Hypersonic Air Breathing Vehicles
At hypersonic Mach numbers (i.e., greater than Mach 5) a flight vehicle having an air breathing engine requires an engine inlet having a large air-capture region. In addition, hypersonic flight vehicle engine design requires inlet surfaces to be three-dimensionally-swept to reduce aerodynamic drag and friction heating. Three-dimensionally-swept engine inlet designs can produce non-planar flow components requiring a complex and perhaps impractical analysis of a three-dimensional airflow path. This may be contrasted with non-hypersonic supersonic engines having ramp inlet surfaces producing planar flow components allowing a much easier analysis of a two-dimensional airflow path.
Furthermore, two-dimensional flow allows easier direct connect engine component testing (which means testing an engine component by duplicating its input conditions with some apparatus without having to use the upstream engine components to produce such input conditions), such as testing an engine without its large inlet by duplicating the two-dimensional airflow conditions calculated at the engine throat. Finally, a hypersonic engine design may introduce a pitch moment on the flight vehicle which would require a constant trim such as from a drag-producing control surface.
Hypersonic aircraft may utilize a combination of turbojet engines and RAMjet or SCRAMjet engines. The turbojet engines are used at relatively low speeds and the RAMjet or SCRAMjet engines are used at relatively high speeds. For example, conventional hypersonic aircraft may use turbojets for flight up to approximately Mach 3 and RAMjets or SCRAMjets for flight at higher speeds.
In RAMjet engines, thrust is produced by passing hot exhaust from the combustion of a fuel through a nozzle. The nozzle accelerates the flow, and the reaction to this acceleration produces thrust. In a RAMjet, high pressure is produced by "ramming" external air into the combustor using the forward speed of the vehicle. SCRAMjet is an acronym for Supersonic Combustion RAMjet. SCRAMjets differ from RAMjets in that combustion takes place at supersonic air velocities through the engine. Since there are no compressors in RAMjets or SCRAMjets, they tend to be lighter and simpler than turbojets, which require a compressor to generate high pressure in the combustor. Since RAMjets and SCRAMjets cannot produce static thrust, other propulsion systems such as turbojet engines must be used to accelerate the vehicle to a speed where the RAMjets or SCRAMjets begin to produce thrust.
RAMjets and SCRAMjets typically include compression ramps at their inlets and expansion ramps at their outlets in order to provide the desired gas pressures entering and leaving the engines. Some hypersonic aircraft engines are equipped with movable inlet ramps.
The nozzles of hypersonic engines are significantly different from conventional nozzle concepts. This is because of the much larger variation range of the decisive parameters. Particularly, the nozzle throat area must be varied at a ratio of 1:5. Further, the existing nozzle pressure ratio, which during operation rises from approximately 3 during take-off to a magnitude of 1,000 at hypersonic flight Mach number 7, thus in principle requires an enormously high variation range of the divergence.
The extremely high divergence is required at hypersonic flight Mach numbers because of the existing high nozzle pressure ratios, i.e., the ratio of the exhaust surface to the nozzle throat surface. The extremely high divergence cannot be implemented inside the nozzle. Therefore, in any case, an afterexpansion path is required which follows the nozzle and is created by the corresponding design of the airplane rear.
The known axially symmetrical convergent/divergent nozzles having a lamellar construction, as used, for example, in military afterburner engines, have a variation range of the nozzle throat area and the divergence which is much too small. Therefore, this type of nozzle cannot be used for the engines of the above-mentioned type.
In addition, convergent, axially symmetrical nozzles with axially displaceable central bodies are known where the throat surface can be adjusted within a wide range. So far, nozzles of this type have been used only in cases with three engines without any afterburning. This is because the cooling of the central body by air taken, for example, from the turbo-engine, presents problems.
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